linux-kernel - [PATCH] mm: fix pfn calculation mistake in __ioremap_check

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Date:   Mon,  7 Jun 2021 17:19:39 +0800
From:   Yaohui Wang <yaohuiwang@...ux.alibaba.com>
To:     dave.hansen@...ux.intel.com
Cc:     luto@...nel.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, yaohuiwang@...ux.alibaba-inc.com,
        luoben@...ux.alibaba.com, Yahui Wang <yaohuiwang@...ux.alibaba.com>
Subject: [PATCH] mm: fix pfn calculation mistake in __ioremap_check_ram

According to the source code in function
arch/x86/mm/ioremap.c:__ioremap_caller, after __ioremap_check_mem, if the
mem range is IORES_MAP_SYSTEM_RAM, then __ioremap_caller should fail. But
because of the pfn calculation problem, __ioremap_caller can success
on IORES_MAP_SYSTEM_RAM region when the @size parameter is less than
PAGE_SIZE. This may cause misuse of the ioremap function and raise the
risk of performance issues. For example, ioremap(phys, PAGE_SIZE-1) may
cause the direct memory mapping of @phys to be uncached, and iounmap won't
revert this change. This patch fixes this issue.

In arch/x86/mm/ioremap.c:__ioremap_check_ram, start_pfn should wrap down
the res->start address, and end_pfn should wrap up the res->end address.
This makes the check more strict and should be more reasonable.

Signed-off-by: Ben Luo <luoben@...ux.alibaba.com>
Signed-off-by: Yahui Wang <yaohuiwang@...ux.alibaba.com>
---
 arch/x86/mm/ioremap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 9e5ccc56f..79adf0d2d 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -74,8 +74,8 @@ static unsigned int __ioremap_check_ram(struct resource *res)
 	if ((res->flags & IORESOURCE_SYSTEM_RAM) != IORESOURCE_SYSTEM_RAM)
 		return 0;

-	start_pfn = (res->start + PAGE_SIZE - 1) >> PAGE_SHIFT;
-	stop_pfn = (res->end + 1) >> PAGE_SHIFT;
+	start_pfn = res->start >> PAGE_SHIFT;
+	stop_pfn = (res->end + PAGE_SIZE) >> PAGE_SHIFT;
 	if (stop_pfn > start_pfn) {
 		for (i = 0; i < (stop_pfn - start_pfn); ++i)
 			if (pfn_valid(start_pfn + i) &&
-- 
2.25.1